The present invention relates to a power supply, and more particularly to a power supply having multiple output voltages of which one or more output voltages may be selectively disconnected from the circuit that they are powering.
It is known to use two separate power converters to supply power to an electronic system. In this known case, the electronic system has a first converter which is of the switching power supply type with multiple voltage outputs. As is common in many switching type converters, the voltage outputs may be disconnected or turned OFF by disabling a pulse width modulated drive signal that drives the power switching transistors to provide the output voltage level. An externally accessible switch logically enables or disables the pulse width modulating signal to provide the switching of the power transistors.
This known case also has a second converter which is of the half wave rectifier type. The second converter has a transformer with a single primary winding and a single secondary winding. The primary winding is connected to an a. c. line voltage and the secondary winding is connected to a half wave rectifier. The half wave rectifier converts the transformed a. c. voltage and current into pulsating d. c. voltage and current. The pulsating d. c. voltage and current are smoothed by some type of filter, such as a inductor-capacitor filter or a simple capacitor filter. The smoothed d. c. voltage and current are then supplied as outputs via conductors attached across the filter capacitor.
The electronic system, that these two converters supply with power, has a CMOS random access memory. Since this CMOS RAM contains program and/or other important information, it is desirable to make the CMOS RAM non-volatile by the connection of an uninterrupted supply of voltage from the previously described, second converter or a back-up battery. The second converter of this electronic system is designed to be continuously connected to the CMOS RAM to maintain the data therein. To this end, the switching converter and its multiple outputs are designed to be turned OFF, for example at the end of the business day, but the program and other important information are designed to be safely maintained via continuously powering the CMOS RAM of the system.
The battery is included to provide back-up power during a power outage. Upon such an outage, the battery powers the RAM until a. c. line power is restored or the battery is discharged. Thus, the second converter, while it is operating, keeps the battery from discharging. The second converter thereby preserves the power of the battery for those emergency situations when it is required.
The problems of this known two converter supply arrangement are that the two separate converters are expensive to manufacture, expensive to connect, and occupy excessive amounts of printed circuit board area. The solution to these problems must include the condition that any replacement converter power supply design has the same ability to switchably disconnect a set of multiple outputs (i.e. turn them OFF) and to continuously power a RAM as the known two converter power supply has.